1. Field of the Disclosed Embodiments
The disclosure relates to batteries, and in particular, detecting the state of charge of a battery.
2. Introduction
The State of Charge (SOC) of a battery is its available capacity expressed as a percentage of its rated capacity. Knowing the amount of energy left in a battery compared with the energy it had when it was new gives the user an indication of how much longer a battery may continue to perform before it needs recharging. As a battery is used, the SOC decreases unless the battery is recharged.
Conventional methods of determining SOC are cumbersome and less than accurate. In process that involves direct measurement, this process would be easy if the battery could be discharged at a constant rate. The charge in a battery is equal to the current multiplied by the time for which it flowed. Unfortunately there are two problems with this. In all practical batteries, the discharge current is not constant but diminishes as the battery becomes discharged, usually in a non-linear way. Any measurement device may therefore be able to integrate current over time. Secondly, this process depends on discharging the battery to know how much charge it contained. In most applications except perhaps in qualification testing, the user (or the system) needs to know how much charge is in the cell without discharging it.
There is also a process called voltage-based SOC estimation which uses the voltage of the battery cell as the basis for calculating SOC or the remaining capacity. Results can vary widely depending on actual voltage level, temperature, discharge rate and the age of the cell and compensation for these factors may be provided to achieve a reasonable accuracy. However, problems can occur with some cell chemistries, particularly lithium which exhibits only a very small change in voltage over most of the charge/discharge cycle.
Current-based SOC estimation is another conventional process that measures the current entering and leaving the cells as a basis for remaining capacity calculation. The charge transferred in or out of the cell is obtained by accumulating the current drain over time. This method, known as Coulomb counting, provides higher accuracy but it still needs compensation for the operating conditions as with the voltage based method. The simplest method of determining the current is by measuring the voltage drop across a low ohmic value, high precision, series and sense resistor. This process of measuring current causes a slight power loss in the current path and also heats up the battery. Hall effect and magneto-resistive transducers avoid this problem but they are more expensive. The former is inaccurate for low currents and the latter can not tolerate high currents and is susceptible to noise. Coulomb counting depends on the current flowing in external circuits and does not take account of self discharge currents. This can be the source of accumulating errors unless the monitoring circuit is regularly reset or calibrated
As batteries are non-linear, voltage characteristics are a function of discharge rate, temperature and age. In another conventional technique, the individual cell model can be constructed from a set of “look-up tables” which provide stepwise approximations of the performance response curves which represent the cell discharge performance as a function of temperature, discharge rate or other parameters. The necessary look-up tables are developed from laboratory measurements under controlled conditions and may be different for each cell chemistry and cell construction. However, this process requires large amounts of data and large numbers of look up tables. As the battery begins to age, the models based on look-up tables model begin to lose effectiveness.